With regard to the closer prior art, the following publications are mentioned, in which polymeric plastics materials are proposed as an addition to POM in order to optimise the heat stability thereof in this way:
U.S. Pat. No. 2,993,025 (synthetic polyamides),
U.S. Pat. No. 3,204,014 (N-vinylpyrrolidone/acrylamide copolymers),
U.S. Pat. No. 3,210,322 (carboxyl group-containing polyamides, polyurethanes, substituted polyacrylamides, polyvinylpyrrolidones and hydrazines),
U.S. Pat. No. 3,215,671 ((meth)acrylates and (meth)acrylamides as crosslinking agents),
U.S. Pat. No. 4,464,435 (copolymers of polyesters, (meth)acrylates, (meth)acrylamides, triallyl cyanurate, diallyl phthalate, vinyl acetate and divinylbenzene),
JP-B-14329/68 (copolymers of acrylamide or acrylamide derivatives with alkyl acrylates, vinyl ethers or vinyl ketones or copolymers of acrylamide derivatives and styrene),
U.S. Pat. No. 3,960,984 (amide oligomer),
U.S. Pat. No. 4,098,843 (dispersion of a polyamide in a carrier resin),
JP-B-22669/68 (ethylene/vinyl acetate copolymers),
EP-A-0 245 962 (polymers and/or oligomers which contain hydroxyl groups and at least one further functional group with a stronger Lewis basicity than the hydroxyl group),
BE-A-722 268 (polyamides, polyurethanes, polyureas, polyvinylpyrrolidone, poly(meth)acrylamide, urea derivatives, amides, hydrazones, semicarbazones and alkylene-bis-phenols),
JP-B-17107/67 (copolymer of acrylamide with styrene or vinylnaphthalene),
EP-A-0 270 278 (mixture of a superpolyamide and a small amount of a transition metal salt),
EP-A-0 270 279 (mixture of a superpolyamide and a small amount of a cyclic amidine compound), and
DE-B-25 40 207 (precipitation polycondensate of formaldehyde and melamine in a molar ratio of between 1.2:1 and 10:1).
It is furthermore known to use vinyl polymers with amide or lactam groups to improve the heat stability.
EP-A-0 381 943 discloses polyoxymethylene moulding compositions which comprise, as an additive, either
or
The polymeric additives according to EP-A-0 381 943 are therefore copolymers which are obtained according to the instructions in EP-A-0 381 943 either by the route of the emulsion polymerization method or by the suspension bead polymerization method.
Just the large number of additives proposed to date reveals that it has not yet been possible to solve the problem of improving the heat stability of reinforced or non-reinforced polyoxymethylene moulding compositions to the complete satisfaction of those in practice. For some applications, known polyoxymethylene moulding compositions still have an inadequate heat stability, which adversely influences processing to shaped articles and, for example, contributes towards deposits on the mould or impairment of the ease of release from the mould and/or leads to discolorations and to a reduction in the mechanical properties during later use of the shaped articles. It is furthermore a disadvantage that the polyoxymethylene moulding compositions known to date may still contain formaldehyde adducts, which lead to odour nuisances during processing at elevated temperatures due to formaldehyde being split off.
Even the more recent additives proposed, for example, in EP-A 0 381 943, which are based on a copolymer or blend of (meth)acrylate/(meth)acrylamide, have so far not found acceptance in practice, since the emulsion polymers incorporated into the polyoxymethylene moulding compositions probably showed no reproducible results in respect of the heat stabilization. Furthermore, the auxiliaries remaining in the emulsion polymer which is to be used to stabilize the POM lead to discolorations or even to decomposition of the polyoxymethylene moulding composition at the usual polyoxymethylene moulding composition processing temperatures of greater than 200.degree. C.